A THz (tera-hertz) wave is an infrared ray having a wavelength of 30 μm to 300 μm and an ultra-high frequency electromagnetic wave having a frequency 1 THz or more.
The THz wave is expected to be widely used in the fields of basic science, engineering, and medicine-biology. For example, in the field of basic science, the THz wave is used in a structure elucidation of physical and chemical materials using a spectroscopy, a discovery of interstellar materials by radio wave frequency and infrared astronomy and spectroscopy science, and the like. In the field of engineering, the THz wave is used in a THz wave ultra-high-speed communication (cosmic space communication and the like), a geo-environmental sensing (a detection of various gases and environmental materials and the like), an assessment of a free electron density and mobility of a semiconductor material, an assessment of an dielectric characteristic of a dielectric functional material, a cubic structure assessment of a organic functional polymer, and internal check of an electronic package. In the field of medicine-biology, the THz wave is used in analysis in a structure of biological functional protein, an imaging of functions of body tissues, and the analysis of a structure of a medication (opaque powder base).
As a method of generating a THz wave, a method using difference frequency generation (DFG) shown in FIG. 1(a) or a method using a parametric oscillation shown in FIG. 1(b) is known.
In the method using the difference frequency generation, by using nonlinear optical crystals (also referred to as a DFG crystals) formed of LiNbO3, GaAs, organic DAST 4-dimethylamino-N-metyl-4-stilbazolium tosylate), and the like as a THz wave generation element, two light waves having different frequencies are inputted to the THz wave generating element to generate the THz wave by using a difference frequency wave which is a difference in frequency between the two light waves.
In this difference frequency generation method, two light sources having different frequencies (ω1, ω2) are required and it is necessary to keep a difference in a frequency between both light sources constant in order to stably generate a THz wave (ω3=ω2−ω1). Therefore, frequencies (wavelengths) of the light waves generated from the two light sources needs to be maintained and controlled in predetermined values with high precision.
Even if the frequency of the THz wave varies by setting one of the two light sources as a wavelength variable light source, a variation precision of the frequency of a wavelength-variable light source is approximately 1 GHz. Accordingly, the frequency control of the THz wave is limited.
Meanwhile, in the method using the parametric oscillation, as disclosed in Patent Document 1 below, the nonlinear optical crystals as the THz wave generating element is disposed in a resonator and one light wave having a predetermined frequency (ω1) is inputted to the THz wave generating element, thereby, the THz wave (ω3, ω1=ω2+ω3) corresponding to a difference between the frequency of an incident light wave and a resonance frequency (ω2) in the resonator by a parametric effect is generated.
[Patent Document 1] Japanese Unexamined Patent Application No. 2002-72269
In this parametric oscillation method, it is advantageous that only one light source is required in comparison with the difference frequency generating method, but the frequency (wavelength) of the light wave generated from the light source still needs to be maintained and controlled in the predetermined value with high precision.
Since resonance efficiency of the resonator needs to be maintained high, a location and a shape of a reflective member (HR mirror) constituting the resonator should be maintained and controlled with high precision.
The present applicant proposed a THz wave generator which stably and efficiently generates a THz wave, and particularly, which is stable with a variation in frequency of a laser light source and can easily vary the frequencies of the THz waves, as described in Patent Document 2 to solve the problem.
[Patent Document 2] PCT/JP2005/5327 (Date of Application: Mar. 31, 2005)
In Patent Document 2, as shown FIG. 2, in a THz wave generator inputting a laser beam from a light source unit (a) including a laser light source to a THz wave generating element (d) and generating a THz wave (fT) from the THz wave generating element, a light circulating unit (b) including an SSB optical modulator and wavelength selecting means (c) are disposed between the laser source unit a and the THz wave generating element (d), the laser beam is inputted to the light circulating unit, a particular wavelength (f0, fn) is selected out of light waves having a plurality of wavelengths (f0, f1 . . . fn . . . ) generated from the light circulating unit by the wavelength selecting means, and the light wave with the particular wavelength is inputted to the THz wave generating element.
In order to raise the optical power of the THz wave, the optical power of the laser beam inputted to the THz wave generating element is raised by forming an optical amplifier prior to the THz wave generating element (d).
However, when the power of the laser beam is amplified up to 10 mW to several W by the optical amplifier, the power of the THz wave is only several nW and there is problem that the optical amplifier has increase in expense and increase in size to gain the higher power.